Silicon nitride ceramics are particularly useful for applications that require high temperature strength and thermal shock resistance. Previously, high pressure methods were used to fabricate these ceramics. However, the use of high pressures often increased manufacturing costs and limited the geometric shape of the formed component. In order to overcome these barriers, researchers developed the technique of pressureless sintering.
Using pressureless sintering techniques, researchers have been able to fabricate Si.sub.3 N.sub.4 -BAS ceramic composites with high strength and some levels of damage tolerance. However, the industry needs further improvements in fracture toughness and flexural strength before pressureless sintered Si.sub.3 N.sub.4 -BAS ceramic composites can rival Si.sub.3 N.sub.4 -BAS composites fabricated using high pressure techniques.
At present time, researchers have been unable to fabricate pressureless sintered Si.sub.3 N.sub.4 -BAS composites that exhibit both increased flexural strength and fracture toughness. Experimentation has shown that Si.sub.3 N.sub.4 -BAS composites with relatively uniform microstructures, consisting of .beta. whiskers of several microns in length, usually exhibit high flexural strength and little or no R-curve behavior. Further, experimentation has also shown that Si.sub.3 N.sub.4 -BAS composites with microstructures consisting of .beta. whiskers with increased diameters usually exhibit high fracture toughness and rising R-curves. Based on these observations, researchers hypothesized that a Si.sub.3 N.sub.4 -BAS composite with optimum distributions of coarse .beta.-Si.sub.3 N.sub.4 whiskers, surrounded by a matrix of short .beta.-Si.sub.3 N.sub.4 whiskers and fine BAS grains, or a bimodal microstructure, would provide an ideal compromise between toughness and strength.
Therefore, what is needed is a composition and a method to fabricate pressureless sintered Si.sub.3 N.sub.4 -BAS ceramic composites, with optimum distributions of coarse .beta.-Si.sub.3 N.sub.4 whiskers, surrounded by a matrix of short .beta.-Si.sub.3 N.sub.4 whiskers and fine BAS grains, that exhibit improved fracture toughness and flexural strength.